Why don't you wait and see the weather forecast a few days before you come? Noone can really tell what the weather will be like in 3 - 4 weeks. It can be chilly, but it can be quite warm too.
An average temperature at that time could be in Athens 7-18 celcious and in Thessaloniki 5-16 celcious but this can easily change as weather at this time of the year changes easily. Dress in layers so you can add or take off depending on the weather. If you are going some place on the mountains you should expect lower temperatures.
Good info:) You will definetely notice a temp drop going up North..more so if you go into the mountains/villages away from Thessaloniki....beautiful territory up there....You won't need bug repellant at that time..it's not hot enough yet for them to 'do their thing':) Have a wonderful time and Enjoy!
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